1. Field of the Invention:
The invention relates in general to elevator systems, and more specifically to elevator systems which produce distance pulses responsive to car movement, and which maintain car position in terms of a count which is incremented and decremented by the distance pulses.
2. Description of the Prior Art:
U.S. Pat. No. 3,750,850, which is assigned to the same assignee as the present application, discloses an elevator system in which an electrical distance pulse or signal is generated in response to each predetermined increment of car travel, such as a distance pulse for each 0.25 inch of car travel. A car position counter is zeroed at the lowest point of car travel, and it is incremented and decremented by the distance pulses, according to travel direction. The car location, in terms of the distance count, when the car is level with each floor of the associated building, is assigned to the associated floor as its address. The car position count, suitably advanced by the normal slowdown distance, is compared with the floor addresses to determine when the car should initiate the slowdown phase of a run to stop at a target floor. The speed pattern generator also uses the car position count to generate the slowdown speed pattern by using the difference between the car position count and the address of the target floor to determine the distance-to-go (DTG) count. The DTG count is the distance from the elevator car to the target floor in terms of the standard increment.
To eliminate the possibility of inaccurate landings due to errors in the car position count, the floor addresses, or in the digital-to-analog conversion in the speed pattern generator, the slowdown pattern is switched to a landing pattern at a predetermined distance from the target floor. The landing pattern is not distance pulse derived, but is an analog signal developed in response to actual car location relative to the target floor by a hatch transducer. Signal blending, such as disclosed in U.S. Pat. No. 3,651,892, which is assigned to the same assignee as the present application, may be used to accommodate mismatches between the slowdown and landing patterns. Calibration of the square root device on each run may also be used to reduce mismatch at the low speed pattern transfer point between the slowdown and landing patterns, as disclosed in U.S. Pat. No. 3,747,710, which is also assigned to the same assignee as the present application.
While the prior art arrangements produce smooth, accurate landings, it would be desirable to be able to eliminate the costly hatch transducer, and to use a distance pulse derived speed pattern all the way to floor level. The problem with such an arrangement is that it is possible for several counts to be lost during a run. For example, the count circuitry may lose or gain counts because of noise during counting, and also noise during transmission of the information to the car controller. If the pulse wheel count is wrong by several counts, the DTG count will differ by the same number of counts, and thus the generated speed pattern will be incorrect. The car may still be able to land at the correct target floor, but it may overshoot or undershoot the landing. If the car overshoots the floor level, it will be forced to relevel, delaying landing. If it undershoots, the elevator car will take too much time to land and become level. For example, at 2 FPM final speed, each two counts lost will delay landing by 1.25 seconds.